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Proportional compasses (sliding pivot in slotted legs)

How do proportional compasses scale a measurement? · Hardware · basis: Xem Studio design following Brown No. 409 (1868)

Also called: proportional compasses, proportional dividers, reduction compass, scaling dividers, compás de reducción, Reduktionszirkel

Exact dimensions. True size, with no printing tolerance added.

sizes
4 in table

Notes

  • Two pointed legs cross on a pivot that sits in a slide block running in a slot in each leg: the points at the two ends open in the ratio of the arm lengths either side of the pivot, so a distance taken with one pair of points is copied at a fixed scale by the other.
  • Drawn with the pivot 26.67 mm from the legs' centres: arms 106.67 and 53.33 mm, ratio 2 : 1, legs at 20°. Moving the slide to the centre gives 1 : 1; a set screw (not drawn) locks it.
  • Brown's No. 409 (proportional compasses with a sliding pivot and scale).
  • Parts (each has its own page): proportional-compass-leg-160-slot6, slide-block-12x6, dowel-pins-metric.
  • Brown's movement No. 409 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Proportional compasses size chart

parametervalue
legs2 × 160 × 14 × 5, slotted
pivot offset drawn26.67 mm
ratio drawn2 : 1
slide blocks2 × 12 × 6.2 (catalog)

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Henry T. Brown (1868, public domain)Five Hundred and Seven Mechanical Movements — scan on archive.orgwebPublisher's page; dimensions are facts
507movements.comBrown's movement No. 409: figure, his text and an animationwebPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

proportional-compasses-mechanism-assemblyAssembled mechanism, all parts placed at one pose (STEP keeps each part separate)STEP · STL
Let your AI pull this. Add https://xemstudio.com/mcp to Claude, ChatGPT, Cursor or any MCP client (free, no account, one minute) and it can look up exact sizes, mating fits and free STEP/STL from every chat.
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Plain text: sheet.txt · JSON: /api/geometry?object=proportional-compasses-mechanism

Proportional compasses (sliding pivot in slotted legs), isometric render of a generated part
Render of proportional-compasses-mechanism-assembly from the free STEP; not to scale.

Fits with

Proportional compass leg 160 × 14 × 5, pointed, 6.2 mm slot 100 long, isometric render of a generated part
Proportional compass leg 160 × 14 × 5, pointed, 6.2 mm slot 100 long160.0×14.0×5.0 mm★ 1 file
Slide block 12 × 6.2 mm for a 6.4 mm slot, isometric render of a generated part
Slide block 12 × 6.2 mm for a 6.4 mm slot2 sizes★ 1 file
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325), isometric render of a generated part
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)5 sizes★ 23 files

Questions

How do proportional compasses scale a measurement?

Two pointed legs cross on a pivot that sits in a slide block running in a slot in each leg: the points at the two ends open in the ratio of the arm lengths either side of the pivot, so a distance taken with one pair of points is copied at a fixed scale by the other.